1. Field of the Invention
This invention relates to a process for liquefying coal. More particularly, this invention relates to an improved liquefaction process.
2. Description of Related Art
While the references are believed to afford a greater understanding of the invention described herein, they are not believed to be "prior art" in the sense they teach or suggest such invention.
It is well known in the art that coal can be converted to valuable liquid products by subjecting it to liquefaction in the presence of a suitable liquefaction solvent. The liquefaction may be conducted optionally in the presence of hydrogen and/or with a suitable catalyst.
The product of liquefaction usually comprises a mixtue of liquefied coal extract and undissolved coal residue, including undissolved extractable carbonaceous matter and mineral matter or ash. The undissolved extractable carbonaceous matter and the finely divided mineral matter remaining in the product of the liquefaction step must be separated from the valuable liquefied carbon products before liquefied carbon products are further processed or utilized commercially. The fine minute particle size of the undissolved coal and mineral matter present in the liquefaction product causes severe difficulties in attempting to use conventional solid-liquid separation techniques to remove these solid particles from the liquefied coal.
Thus attempts to use filtration techniques have not been particularly successful, because of plugging of the filter parts with or without a precoat and the expense involved in providing the requisite large filtration area. Similarly, gravity settling techniques also have not been successful, because of relatively low settling rates and inefficient ash removal. Centrifugation techniques have also not met with a great deal of commercial success, because of relatively high cost and the difficulty in separating the lighter finely divided materials.
Attempts have been made in prior art to separate the finely divided ash and undissolved coal particles by chemical means, i.e., by contacting the liquefied coal with a so called promoter liquid which is said to facilitate subsequent gravity settling of the finely divided ash and undissolved coal particles, e.g., as described in U.S. Pat. Nos. 3,852,182 and 3,856,675. However, the use of the promoter liquid described in the aforementioned prior art also has a number of disadvantages, because the process-using the promoter liquid, which is a fraction boiling between 425.degree. and 500.degree. F., also uses a liquefaction solvent comprising a hydrocarbon fraction boiling between 600.degree. and 900.degree. F., which, after liquefaction, is subjected to a conventional distillation separation technique, whereby the promoter liquid and the liquefaction solvent fractions are recovered for subsequent use in the process. During the distillation step, however, the presence of the high-boiling fraction (i.e., of the 600.degree. to 900.degree. F. liquefaction solvent fraction) makes the distillation step susceptible to the production of coke and/or coal which clog up the process equipment and decrease the yield of liquid products. Thus, the liquefaction step must be conducted at increasingly more severe temperature and pressure conditions to produce increased proportions of light liquid products, which are used as the promoter liquid. The use of increasingly more severe temperature and pressure conditions in the liquefaction step, however, results in increased hydrogen consumption and production of large amounts of gas and coke, at the expense of desirable liquid.
Other methods of interest are disclosed in three articles printed in the June 1979 CEP. They are "Antisolvent Deashing" by M. Peluso et al., page 41, "Critical Solvent Deashing of Liquefied Coal" by R. M. Adams et al., page 44 and "Supercritical Extraction of Coal," by R. R. Maddocks et al., page 49.